ARDI's "Executor" was one of the first MacOS/m68k emulation suites available for the IBM-Compatible PC and NeXTSTEP platforms. It is capable of running most MacOS 6 & 7 applications and can handle almost every form of Macintosh formatted disk.
ARDI was able to achieve this in "Executor" without reverse engineering or including binary images of Apple's intellectual property.
This was a commercial product that was eventually released as open source under the MIT License in 2008.
More Details:
http://www.ardi.com/ (circa 1998)